1. Field of the Invention
Embodiments of the invention relate generally to crystal oscillators and more specifically to crystal oscillators with reduced acceleration sensitivity.
2. Description of the Related Art
Quartz crystals are commonly used to control the frequency of electronic oscillators. Although quartz is one of the most stable materials available for fabricating a high frequency resonator, certain limitations can become apparent in precision applications. For instance, changes in the ambient temperature cause the resonant frequency to change. In addition, the natural frequency of a quartz resonator can also be affected by applied acceleration forces. In some situations, these effects on the frequency are relatively small and can go undetected (˜0.0000002% per g of applied force). However, in many applications, an oscillator must operate in an environment which subjects it to levels of vibration or shock, where the resultant frequency shifts can be significant and can limit system performance. These deleterious effects are a well known problem and a major concern of oscillator designers.
Acceleration forces applied to a crystal oscillator assembly will cause a shift in the operating frequency. If these forces are in the form of periodic or sinusoidal vibration, frequency modulation will appear as sidebands to the carrier at the vibration frequency, the amplitude of which is determined by the amount of frequency shift. When the acceleration forces are in the form of random vibration, an increase in the broadband noise floor of the oscillator will occur. Either of these conditions may result in serious degradation of system performance in a noise sensitive application. Shock pulses due to handling or other environmental events can cause a jump in the frequency which may result in circuit malfunction such as loss of lock in phase locked loop or GPS tracking applications.
There are generally two classes of methods to minimize the effects of acceleration forces on crystal resonators. The first class is known as active compensation. In active compensation, an acceleration sensor is used to detect the characteristics of applied forces and a signal is then processed and fed back to the oscillator circuit to adjust the frequency by an equal magnitude but in the opposite direction from the acceleration induced shifts. This method can be effective over certain vibration frequency ranges, but it requires a relatively complex circuit and can be very expensive to implement.
The second class is referred to as passive compensation. Passive methods do not attempt to sense the applied acceleration. Generally, in passive methods, the crystal resonator or resonators are constructed using special methods that render them somewhat insensitive to acceleration forces. Passive methods can be effective, but they generally require an involved and difficult fabrication process to produce the required crystal or composite crystal assembly.
In view of these complications, one attempt has been to cancel acceleration sensitivity including determining a dominant sensitivity axis of the resonators and then mounting the resonators with the dominant sensitivity axes in an anti-parallel arrangement. However, in aligning the resonators according to a supposedly dominant axis, such methods do not take into account the sensitivities along the other axes. As a result, the exact maximum magnitude and direction of a crystal's acceleration sensitivity characteristic is not accounted for in such methods, and it is less effective in minimizing the effects of acceleration forces.